Application Of Sodium Lignosulfonate In Aqueous Graphene And The Determination Of Its Sulfonation Degree

The special structure of graphene endowed it a host of fascinating properties, and it has triggered great attention in its novel application fields. Among the approaches of fabricating high quality graphene, liquid exfoliation exhibited low cost, easy operation, high yield and concentration, which led it promising to manipulation and further application. However, there are two problems to be settled for this technology. Firstly, most organic solvents are toxic and lack of biocompatibility, which limits the safe use of graphene in the field of biomedical application. Secondly, the concentration of aqueous graphene suspensions obtained is rather low for the further purification and application, so it is essential to fabricate aqueous graphene suspension(AGS) of high concentration.As a kind of anionic polymeric dispersants with hydrophobic aromatic backbone with alkyl side chains, sodium lignosulfonate(SL) is abundant, nontoxic and low cost, which presents good dispersion for hydrophobic solid particles in aqueous suspensions. Meanwhile, the sulfonated degree of lignosulfonate is an essential factor for its dispersion performance. However, it existed lots of shortcoming for the traditional methods of determining its sulfonation degree, such as complex process high cost and low accuracy. It is meaningful to apply the sodium lignosulfonate into dispersing aqueous graphene suspensions, and determine its sulfonation degree effectively.A series of dispersants were selected for dispersing aqueous graphene suspensions, and it was found that anionic dispersants possessed particularly remarkable dispersing performance for AGS compared with other dispersants, especially the sodium lignosulfonate. Four SL fractions with different molecular weight(Mw) and sulfonic acid group prepared by ultrafiltration of a commercial SL were applied to evaluate the dispersing mechanism and the effects of SL molecular structure on the AGS, and it was revealed that the maximum concentration of AGS dispersed by the SL fractions increased with the decline of Mw and with the increase of sulfonic acid group. While the CG can be further improved through improving the hydrophobicity of SL by either covalent or non-covalent modification. Meanwhile, the concentration of AGS dispersed by different dosage of SL after ion-exchanged was obviously higher than SL before ion-exchanged, which suggested that high ion strength had a negative impact on dispersing AGS. It was found that the concentrations of AGS were proportional to the zeta potentials, which illustrated that electrostatic repulsion played the most important role during the dispersion of the stabilizing dispersant-coated AGS against aggregation. The structure and performance of the obtained aqueous graphene suspensions were characterized by UV-Vis absorption spectroscopy, Raman micro spectroscopic setup, X-ray photoelectron spectra and dispersion stability instrument, and it was found that stable AGS with high concentration and little defect were prepared through exfoliating from graphite by sonication with the aid of sodium lignosulfonate. The concentration of AGS dispersed by the modified SL can reach up to 13.5 g/L at an initial graphite concentration of 200 g/L, which is substantially higher than those reported previously.A novel method for determining the sulfonation degree of sodium lignosulfonate of different molecular structure was put forward by precipitating it with Hexadecyltrimethyl ammonium Bromide(CTAB). Firstly, the pH of the prepared sodium lignosulfonate solution was adjusted to 5.5, which made the sulfonic acid group was completely ionized while the other hydrophilic functional groups haven’t been ionized. Then a certain amount CTAB solution was mixed with sodium lignosulfonate, when the sulfonic acid group of sodium lignosulfonate electrostatically interacted with the ammonium group of CTAB according to a mole ratio of 1:1, the sodium lignosulfonate was completely precipitated by the CTAB. The obtained sodium lignosulfonate mixture was centrifuged and the absorbance of the supernatant was determined by UV-Vis, and a titration curve could be obtained with different ratio dosage of CTAB. The sulfonation degree of sodium lignosulfonate could be determinate at the minimum point of the titration curve. This technology for determining the sulfonation degree of sodium lignosulfonate by precipitating it with CTAB performed lots of advantages, such as low costs, easy operation and high accuracy, which is of great importance for promoting the development and broadening the application field of sodium lignosulfonate.